(I) The effects of cis and trans unsaturation on the conformation and irregular packing of long fatty acid chains will be studied by low temperature X-ray structure determination of cholesteryl alkenoates. The aim is to formulate structural principles for models of lipid agg regates such as the interior of biomembranes and lipoprotein particles. (II) The dynamical properties of semi-rigid systems, such as the fused rings of cholesterol will be studied by correlating small conformational deformations in many crystal structures with non-rigid anisotropic thermal vibrations. (III) The distribution of charge density and electrostatic potential in simple polar lipids will be determined by low temperature X-ray and neutron diffraction. We will study alkane dicarboxylic acids (HOOC(CH2)nCOOOH, n = 4, 6, 8] and their Li+, Na+, Mg2+ salts. From the simple crystal bilayers, we plan extrapolations to more complex micellar structures characteristic of surfactants.